Conventional bearings for high speed rotating devices are know to be subject to wear, noise, vibration and thermal problems. Until recently, practical magnetic bearings have been of either the permanent magnet or the electromagnet feedback type. Permanent magnet bearings are subject to inherent static instabilities and must be stabilized in at least one degree of freedom by non-magnetic means (e.g. a rotatable coupling). Feedback--based magnetic bearings often require elaborate position sensors and electronics to achieve stability.
The prior art has attempted to improve the magnetic bearing art by turning to superconductivity. In such instances, either the bearing member or the rotating member or both are maintained in a Type I superconducting state so as to achieve a magnetic pressure therebetween and thereby provide a desired degree of levitation. Type I superconductors exhibit perfect diamagnetism up to a critical applied field, at which point superconductivity is lost and the magnetization of the sample rises abruptly. Examples of superconducting bearings of the Type I kind can be found in U.S. Pat. Nos. 3,493,274 to Emslie et al and 3,026,151 to Buchhold. In order to obtain stability in those systems, the bearing structures generally rely on either a mechanical rotary support (e.g. Buchhold) or employ dished or other encompassing type superconductors whereby the shape provides a gravitational minimum which leads to limited lateral stability (see Emslie et al).
Recently, others have discovered new ceramic compositions which exhibit superconducting properties at temperatures in excess of liquid nitrogen. These new superconductors are generally Type II materials with upper critical fields typically greater than 30-35 Teslas. A Type I superconductor may be said to "screen out" magnetic flux from its interior. By contrast, a Type II superconductor enables magnetic flux to penetrate into its interior in clusters of flux lines. Under such circumstances, circulating superconducting currents are established within the Type II superconductor. They, in turn, generate substantial magnetic fields and exert a positional pinning effect on a magnet levitated over the surface of the superconductor.
A benefit to be potentially gained from a levitated superconducting bearing is its ability to achieve rotational speeds of 10's of thousands of rpm. In order to attain such speeds, teachings of the prior art which suggest immersion of the entire unit in a liquid helium/nitrogen environment are impractical. Prior art teachings that call for exquisitely balanced rotors with "hard suspensions" are also to be avoided if possible. Furthermore, external rotational stabilization is to be avoided, if at all possible.
Accordingly, it is an object of this invention to provide a superconducting rotating assembly which exhibits levitated lateral, vertical and axial stability while enabling rotation at high speed.
It is another object of this invention to provide a superconducting rotating assembly of simple and inexpensive design which is adapted for high speed, stable rotation.
It is a further object of this invention to provide a superconducting rotating assembly which employs a soft suspension for accommodating rotors with significant imbalance.